The cornea is the clear, protective outer layer of the eye. It serves as a barrier against dirt, germs, and other particles that can harm the delicate components of the eye. The cornea is also capable of filtering out some amounts of the sun's ultraviolet light, and plays a key role in vision by refracting light that enters the eye. The cornea can be susceptible to damage or impairment through corneal disease or physical injury. Corneal diseases include a variety of conditions that affect mainly the cornea such as infections, degenerations, and many other disorders of the cornea that may arise as a result of heredity.